In order to change the amount of protein in seeds, the following have been conventionally used: (1) an improved cultivation method; (2) a method for processing harvested seeds, and particularly grains such as rice grains, with an acid or bacterium; (3) molecular breeding using markers; (4) mutant screening; (5) gene recombination; and other methods.
Problems relating to the above methods and the object achieved by the present invention are described below.
According to the method (1) above, it is possible to change the protein amount, although it is only possible to increase or decrease the amount to a slight extent. In addition, although the method (2) above is effective to a certain extent for reducing the protein amount, processing of harvested seeds is labor- and time-consuming. Further, advantageous results such as an increase in protein amount cannot be obtained according to the method (2) above. According to the method (3) above, the protein amount is determined to be a quantitative trait. In order to modify such trait by a conventional breeding method, it is necessary to identify a plurality of gene loci that contribute highly to trait expression by QTL analysis, to specify the causative gene at each gene locus, and to introduce each causative gene into a desired variety by crossing. Therefore, the method (3) above is also labor- and time-consuming. With the method (4) above, a low-glutelin rice line such as LGC-1 is bred. However, the amount of remaining gulutelin accounts for 30% to 50% of that in the original variety. In addition, there are problematic points common to low-glutelin rice lines. In fact, the amount of glutelin, which is an easily digestible protein, decreases to significantly below the level found in the original variety. However, this in turn causes a significant increase in the amount of prolamin, which is an indigestible protein. Therefore, the method (4) above cannot be evaluated as a method for reducing total seed protein content. In the case of the method (5) above, it has been reported that the total expression level of the prolamin multigene group was remarkably reduced, resulting in reduction of the protein content in rice seeds (Patent Document 1: WO2004/056993). However, in this case, the decrease in the total protein content is 15% at maximum, although the amount of prolamin itself decreases to 50% or less of the original amount. In addition, regarding the method (5) above, it has been reported that transcription factors specified by AT1G04550, AT1G66390, AT5G13330, and At2g30420 were overexpressed in Arabidopsis thaliana seeds, which resulted in, respectively, 25%, 14%, 39%, and 17% increases in protein content. Also, it has been reported that overexpression of a transcription factor specified by At2g47460 resulted in a decrease in the seed storage protein content of 13% (Patent Document 2: WO 01/35727).
In spite of the development of the above molecular breeding methods for the improvement of a variety of traits, there are still no practically available techniques to increase or decrease seed protein content.
As reasons for the above, it is considered that truly excellent genes remain undiscovered, and that new recombinant varieties that have been confirmed to have desirable effects in the test phase cannot exhibit expected effects upon practical use in different environments. In addition, a number of genes are involved in the expression of quantitative traits such as seed protein content in different steps in the control system, the metabolizing system, and other systems. Thus, it has been difficult to discover or develop truly excellent genes capable of improving quantitative traits. In order to solve such problems, an object of the present invention is to find a novel gene exhibiting remarkably high effects. Another object of the present invention is to develop a gene capable of exerting effects in a practical environment to an extent comparable to the effects exerted in the test phase.